Surprise Find: Fragments of Fallen Asteroid

by Paul Gilster on March 26, 2009

Asteroid 2008 TC3 is surely a sign of progress. The eighty ton asteroid, which made a spectacle of itself upon entry into Earth’s atmosphere on the morning of October 7, 2008, was the first space rock to have been observed before it collided with our planet. What we’re hoping, of course, is that any future objects headed our way will be spotted early enough that, if their size warrants, they can be diverted or destroyed.

It was thought that 2008 TC3 did a good job of destroying itself when it exploded some 37 kilometers above the Nubian desert, but two researchers recently traveled to the Sudan and, with help from students at the University of Khartoum, collected 280 pieces of asteroid over a 29-kilometer field. Peter Jenniskens (SETI Institute) calls the event “…an extraordinary opportunity, for the first time, to bring into the lab actual pieces of an asteroid we had seen in space.” Jenniskens is lead author on the paper that now appears as the cover on the latest Nature.

I mention the significance of detecting objects early, but this one is a reminder of how tough the challenge is, being detected by the Catalina Sky Survey a mere twenty hours before its explosive arrival. We’re going to need longer lead times than that in the event of future emergency, but of course 2008 TC3 was also a tiny object, roughly the size of a truck. It’s also interesting in its own right, as an analysis of its debris shows. So-called F-class asteroids like this one have never before yielded a sample that could be studied in the laboratory. Now it’s giving out potentially useful secrets.

The meteorites it produced — called polymict ureilites — are porous, dark and rich in carbon. Nature is running a gripping account of the fall of 2008 TC3 by Roberta Kwok in the same issue as the Jenniskens paper, from which this snip about the composition of the meteorite:

Jenniskens couriered a sample to Mike Zolensky, a cosmic mineralogist at the NASA Johnson Space Center in Houston, Texas. Examining the rock, Zolensky discovered that it contained large chunks of carbon and glassy mineral grains resembling sugar crystals. Tests at other labs confirmed that the sample was a ureilite, a type of meteorite thought to come from asteroids that have melted during their time in space. Only 0.5% of objects that hit Earth yield fragments in this category. But 2008 TC3’s pieces are strange even for ureilites: they are riddled with an unusually large number of holes, says Zolensky. “It boggles the mind that something that porous could survive as a solid object,” he says.

Mind boggling indeed. And the behavior of 2008 TC3 is also a potent indicator that these meteorites are the fragments of a fragile parent. 2008 TC3’s high-altitude explosion shows that any future F-class asteroid on an Earth-crossing trajectory may be likewise fragile, and thus likely to disintegrate into a lethal rain of debris if simply blown up. Alternative strategies for dealing with such asteroids are under consideration, but knowing which to use on what target is key. That also makes sharpening our asteroid identification skills from afar a continuing priority.

Asteroids could be nudged away from a collision course with Earth by a small nuclear explosion or by using lasers to vaporize a region, creating a plume of gas that should provide enough thrust to push the asteroid off course, according to Lawrence Livermore National Laboratory and University of Glasgow scientists….

Perhaps one way to deal with very large asteriods, those which are about 1 trillion metric tons in mass, would be too detonated an array or planer sheet like arrangement of thousands of 100 kiloton neutron bombs.

The sheet of neutron bombs might be detonated about 1 kilometer from the surface of the asteriod wherein the neutron flux would vaporize a layer of the asteriod surface as great as dozens of meters thick. Note that a hydrogen bomb of a yield of about 15 megatons detonated in the Bikini Atoll essentially vaporized a creator about 7,000 feet accross and about 270 feet deep in the middle. No doubt that some of the effect was not strictly vaporization, but also included mechanical pulverization of the coral rock like material comprising the Island’s surface. However, if a simmilar amount of energy could be deposited in one feld swoop within a side of the asteriod, down to a depth of dozens of meters, the ablative reaction should give the asteriod a pretty darn good shove.

Using 3,000 one hundred kiloton neutron bombs would yield 300 megatons of blast energy. If one tenth of this energy could be deposited within the asteriod down to a depth of tens of meters, about 10 million to 30 million tons of the asteriod’s material should be vaporized thus causing a high pressured gas in that amount to push off on the asteriod. The amount vaporized would depend of course also on the heat capacity of the asteriodal material and its heat of vaporization.

Perhaps the bomb will not destroy us by rather be an instrument to save us.

Explanation: Small asteroid 2008 TC3 fell to Earth at dawn on October 7, 2008, tracking through the skies over the Nubian Desert in northern Sudan.

That event was remarkable because it was the first time an asteroid was detected in space before crashing into planet Earth’s atmosphere. It was generally assumed the asteroid itself had completely disintegrated to dust.

But, based on satellite and ground observations of the atmospheric impact event, Dr. Mauwia Shaddad of the University of Khartoum, aided by Dr. Peter Jenniskens of the SETI Institute and NASA Ames Research Center, led an expedition of students and staff to the area, combing the desert for surviving fragments.

On December 6, 2008, two hours after their search began, the first meteorite was found. The team ultimately collected some 280 small meteorites, now called Almahata Sitta, with a total mass of about 5 kilograms — the first material recovered from a known asteroid.

In stark contrast to the lighter-colored stones, the black fragment in the picture is Almahata Sitta meteorite number 15. About 4 centimeters in diameter, it is seen as it came to rest on the desert floor.

A miniature meteorite unlike any other has been discovered in Antarctica.

The tiny rock, known as MM40, is the first achondritic basaltic micrometeorite ever found on Earth. Detailed analysis shows it has an unusual chemical composition the researchers say raises questions about where it originated in the Solar System and how it was created.

“We have basaltic meteorites that are thought to come from an asteroid called 4 Vesta and we also have basaltic meteorites from the Moon and Mars,” said Dr. Caroline Smith, curator of meteorites at the Natural History Museum, London “But MM04’s chemistry does not match any of those places. It has to be from somewhere else.”

The first asteroid to have been spotted before hitting Earth, 2008 TC3, crashed in northern Sudan one year ago on October 6. Several astronomers have been trying to piece together a profile of this asteroid, pulling together information from from meteorites found at the impact site and the images captured of the object in the hours before it crashed to Earth.

“We have a gigantic jigsaw puzzle on our hands, from which we try to create a picture of the asteroid and its origins,” said SETI Institute astronomer Peter Jenniskens, who worked at the crash site, “and now we have with a composite sketch of the culprit, cleverly using the eyewitness accounts of astronomers that saw the asteroid sneak up on us.” Their description? 2008 TC3 looked like a loaf of walnut-raisin bread.

“The asteroid now has a face,” says SETI Institute astronomer Peter Jenniskens, chair of the special session. Last December, Jenniskens and Sudan astronomer Muawia Shaddad went to the crash site and recovered 300 fragments in the Nubian Desert. Like detectives, students from the University of Khartoum helped sweep the desert to look for remains of the asteroid. They found many different-looking meteorites close to, but a little south, of the calculated impact trajectory.

Charter

In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last seven years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

On Comments

Centauri Dreams publishes selected comments on the articles under discussion here. Among the criteria for selection: Comments must be on topic, directly related to the post in question, must use appropriate language and must not be abusive to others. Civility counts. In addition, a valid email address is required for a comment to be considered. Centauri Dreams is emphatically not a soapbox for political or religious views submitted by individuals or organizations.